Difficult to answer without knowing what values are actually in X1/W1 when
you are getting th 19:00 result.

In the meantime, I think you can simplify your formula to:
=IF(W1="","w1 empty",IF(W1<X1,X1-W1,""))
or (to put it more like yours is now)
=IF(W1="","",IF(W1<X1,X1-W1,""))

"default time to PM after 12:00 pm" wrote:

I'm trying to calculate the hh:mm difference between two cells and asking to
do nothing if W1 is blank. Why am I getting 19:00 as a value from a
calculation of:

=IF(W1<X1,X1-W1,IF(W1="","",IF(W1X1,"","")))